Input redirection with QtQuick window thumbnails

In the overview effect, it can be desired to forward mouse events to the panel. On X11, it's a difficult problem. On the other hand, on Wayland, it should be doable as kwin is in charge of input handling. We may need to tweak some internal abstractions though, but it still should be technically possible.

Implementation-wise, we need to override various input related methods in QQuickItem, e.g. QQuickItem::mousePressEvent(), QQuickItem::hoverMoveEvent(), etc.

We may potentially hit the problem with losing some input event data. For example, the MouseEvent type contains all of QMouseEvent data but also extra stuff, e.g. unaccelerated delta, etc.

This also begs a question regarding whether the scene should be in charge of deciding what focus object is.

This task is good opportunity to clean up existing input abstractions in kwin. For example, it would be nice to unify focus handling for regular and internal windows, i.e. kill InputDeviceHandler::internalWindow(), etc.